Many kinds of technique have been recently proposed to switch the combustion state in engine cylinders in accordance with the traveling state of a vehicle, that is, to perform both premixed combustion and diffusion combustion in one engine. The premixed combustion is a state of burning a premix of fuel and oxygen (oxidant) in, for example, a spark-ignition gasoline engine that injects fuel into an intake port. The diffusion combustion is a state of burning a somewhat inhomogeneous mixture of fuel and oxygen (combustion with diffusion of fuel and oxygen) in, for example, a compression-ignition diesel engine that injects fuel directly into the cylinders.
Diesel engines achieving these two burning states are called premixed charge compression ignition (PCCI) engines, homogeneous charge compression ignition (HCCI) engines, and controlled auto ignition (CAI) engines. Another technique has been also developed today in order to achieve the two burning states in one spark-ignition gasoline engines. This technique conduces to premixed compression self-igniting combustion in the gasoline engines.
The premixed combustion, combustion of a lean mixture, has a lower combustion temperature and emits less NOx (nitrogen oxide) and soot than the diffusion combustion. Achieving the premixed combustion in the engine of a vehicle of which NOx and soot emission is regulated improves the emission performance of the vehicle. (See Japanese Patent No. 5447294 and No. 3931900.)